There is a growing need for large-scale energy storage systems that can support electrical transmission grids and enable the reliable implementation of intermittent renewable sources. Of particular interest are redox flow batteries, which are rechargeable electrochemical energy storage devices that utilize the oxidation and reduction of two soluble electroactive species for charging (storing energy) and discharging (releasing energy). The redox active species are stored in separate liquid reservoirs and pumped to and from the power converting device (e.g., electrochemical cell stack). This differs from conventional enclosed secondary batteries, such as lithium ion batteries, where, during cycling, the working ion shuttles between two solid electrode structures which contain the redox active materials. The energy capacity of a redox flow battery is generally independent of electrode or stack size, and can be scaled for a given electrolyte simply by changing the volume of the liquid electrolytes. The energy density of the electrolyte, and thus the overall system, generally depends on the concentration of the redox active species in the electrolyte and/or on the number of electrons each species can transfer during operation.
Known aqueous redox flow battery chemistries include: iron-chromium, bromine-polysulfide, vanadium-bromine, all vanadium, zinc-bromine, and soluble lead-acid. Commercialization of these technologies have been limited by characteristics inherent to the chemical systems, including: low energy density, low round-trip energy efficiencies, and high costs. For most systems the electrolytes themselves represent a significant fraction of overall cost. The low cost of water relative to other solvents or solid electrode designs make aqueous redox flow batteries an attractive option if combined with redox active materials that can offer complimentary advantages including high solubility, a high open-circuit voltage, long-term durability, and low material costs. Many existing aqueous flow batteries operate in acid or base, which require expensive materials of construction for piping, tanks, and electrochemical cell components. A battery that operates at milder conditions while maintaining similar energy density to more caustic systems may be advantageous for durability reasons alone.
The energy density of most aqueous flow batteries are limited by both the solubility of the redox active material, typically less than 2 M, and the number of electrons transferred, typically 1 per molecule. Most aqueous flow battery technologies are based on transition metal redox active species. These species primarily utilize a single electron transfer for the reactions at both the positive and negative electrode, which limits the theoretical charge concentration of an electrolyte to the species concentration. Many organic species are highly soluble in aqueous electrolytes and are capable of undergoing 2 electron transfer. If both the positive and negative species are soluble enough to enable a charge carrier concentration greater than 2 M, they could enable significantly higher energy density than the presently-employed aqueous flow batteries. Moreover, these organic species may be more easily-synthesized than the typical metal salts in terms of required energy input and environmental impact, and/or operation at mild pH may reduce costs on balance of plant materials, possibly replacing stainless steel with plastics. This would enable systems with smaller footprints and reduced material costs to achieve the same power output and energy storage capability. These flow batteries could be located almost anywhere in the transmission grid or in a distribution system and could significantly help stabilize the grid in critical or remote locations and relieve transmission congestion. Load leveling helps the environment by allowing a fossil fuel power plant to operate at its optimum efficiency level.
There is an ongoing need for new, more efficient, aqueous redox flow batteries. Accordingly, improved materials, systems, and methods are needed.